"Male koi are slender, compact and torpedo-like in appearance, with very trim lines and smooth body contours. They also show behaviour that is particular to male koi and is particularly noticeable when all male koi are segregated into a single tank. Male koi tend to dart around energetically, with sharp turns of speed, showing real 'attitude' and a skittish character.

Females on the other hand, do not develop tubercles nor will they retain the sleek lines of a male as the breeding season approaches. Eggs that develop in the ovaries will swell, leading to an increase in volume to the abdomen. In addition, female koi tend to behave in a less timid manner to a tank full of males, feeding voraciously as soon as the food hits the water."